如何声明、实例化和使用委托（C# 编程指南）

项目
11/09/2022

可以使用以下任一方法声明委托：

使用匹配签名声明委托类型并声明方法：

// Declare a delegate.
delegate void Del(string str);

// Declare a method with the same signature as the delegate.
static void Notify(string name)
{
    Console.WriteLine($"Notification received for: {name}");
}

// Create an instance of the delegate.
Del del1 = new Del(Notify);

将方法组分配给委托类型：

// C# 2.0 provides a simpler way to declare an instance of Del.
Del del2 = Notify;

声明匿名方法：

// Instantiate Del by using an anonymous method.
Del del3 = delegate(string name)
    { Console.WriteLine($"Notification received for: {name}"); };

使用 lambda 表达式：

// Instantiate Del by using a lambda expression.
Del del4 = name =>  { Console.WriteLine($"Notification received for: {name}"); };

有关详细信息，请参阅 Lambda 表达式。

下面的示例演示如何声明、实例化和使用委托。 BookDB 类封装用来维护书籍数据库的书店数据库。它公开一个方法 ProcessPaperbackBooks，用于在数据库中查找所有平装书并为每本书调用委托。使用的 delegate 类型名为 ProcessBookCallback。 Test 类使用此类打印平装书的书名和平均价格。

使用委托提升书店数据库和客户端代码之间的良好分隔功能。客户端代码程序不知道如何存储书籍或书店代码如何查找平装书。书店代码不知道它在找到平装书之后对其执行什么处理。

示例

// A set of classes for handling a bookstore:
namespace Bookstore
{
    using System.Collections;

    // Describes a book in the book list:
    public struct Book
    {
        public string Title;        // Title of the book.
        public string Author;       // Author of the book.
        public decimal Price;       // Price of the book.
        public bool Paperback;      // Is it paperback?

        public Book(string title, string author, decimal price, bool paperBack)
        {
            Title = title;
            Author = author;
            Price = price;
            Paperback = paperBack;
        }
    }

    // Declare a delegate type for processing a book:
    public delegate void ProcessBookCallback(Book book);

    // Maintains a book database.
    public class BookDB
    {
        // List of all books in the database:
        ArrayList list = new ArrayList();

        // Add a book to the database:
        public void AddBook(string title, string author, decimal price, bool paperBack)
        {
            list.Add(new Book(title, author, price, paperBack));
        }

        // Call a passed-in delegate on each paperback book to process it:
        public void ProcessPaperbackBooks(ProcessBookCallback processBook)
        {
            foreach (Book b in list)
            {
                if (b.Paperback)
                    // Calling the delegate:
                    processBook(b);
            }
        }
    }
}

// Using the Bookstore classes:
namespace BookTestClient
{
    using Bookstore;

    // Class to total and average prices of books:
    class PriceTotaller
    {
        int countBooks = 0;
        decimal priceBooks = 0.0m;

        internal void AddBookToTotal(Book book)
        {
            countBooks += 1;
            priceBooks += book.Price;
        }

        internal decimal AveragePrice()
        {
            return priceBooks / countBooks;
        }
    }

    // Class to test the book database:
    class Test
    {
        // Print the title of the book.
        static void PrintTitle(Book b)
        {
            Console.WriteLine($"   {b.Title}");
        }

        // Execution starts here.
        static void Main()
        {
            BookDB bookDB = new BookDB();

            // Initialize the database with some books:
            AddBooks(bookDB);

            // Print all the titles of paperbacks:
            Console.WriteLine("Paperback Book Titles:");

            // Create a new delegate object associated with the static
            // method Test.PrintTitle:
            bookDB.ProcessPaperbackBooks(PrintTitle);

            // Get the average price of a paperback by using
            // a PriceTotaller object:
            PriceTotaller totaller = new PriceTotaller();

            // Create a new delegate object associated with the nonstatic
            // method AddBookToTotal on the object totaller:
            bookDB.ProcessPaperbackBooks(totaller.AddBookToTotal);

            Console.WriteLine("Average Paperback Book Price: ${0:#.##}",
                    totaller.AveragePrice());
        }

        // Initialize the book database with some test books:
        static void AddBooks(BookDB bookDB)
        {
            bookDB.AddBook("The C Programming Language", "Brian W. Kernighan and Dennis M. Ritchie", 19.95m, true);
            bookDB.AddBook("The Unicode Standard 2.0", "The Unicode Consortium", 39.95m, true);
            bookDB.AddBook("The MS-DOS Encyclopedia", "Ray Duncan", 129.95m, false);
            bookDB.AddBook("Dogbert's Clues for the Clueless", "Scott Adams", 12.00m, true);
        }
    }
}
/* Output:
Paperback Book Titles:
   The C Programming Language
   The Unicode Standard 2.0
   Dogbert's Clues for the Clueless
Average Paperback Book Price: $23.97
*/

可靠编程

声明委托。

以下语句声明新的委托类型。
```
public delegate void ProcessBookCallback(Book book);
```
每个委托类型描述自变量的数量和类型，以及它可以封装的方法的返回值类型。每当需要一组新的自变量类型或返回值类型，则必须声明一个新的委托类型。
实例化委托。

声明委托类型后，则必须创建委托对象并将其与特定的方法相关联。在上例中，你通过将 PrintTitle 方法传递给 ProcessPaperbackBooks 方法执行此操作，如下面的示例所示：
```
bookDB.ProcessPaperbackBooks(PrintTitle);
```
这将创建一个新的与静态方法 Test.PrintTitle 关联的委托对象。同样，如下面的示例所示，传递对象 totaller 中的非静态方法 AddBookToTotal：
```
bookDB.ProcessPaperbackBooks(totaller.AddBookToTotal);
```
在这两种情况下，都将新的委托对象传递给 ProcessPaperbackBooks 方法。

创建委托后，它与之关联的方法就永远不会更改；委托对象是不可变的。
调用委托。

创建委托对象后，通常会将委托对象传递给将调用该委托的其他代码。委托对象是通过使用委托对象的名称调用的，后跟用圆括号括起来的将传递给委托的自变量。下面是一个委托调用示例：
```
processBook(b);
```
委托可以同步调用（如在本例中）或通过使用 BeginInvoke 和 EndInvoke 方法异步调用。

如何声明、实例化和使用委托（C# 编程指南）

示例

可靠编程

另请参阅